Being located at the very toe end of Cornwall, less than 2 miles from Land's End, Sennen picks up as much swell as any North Coast beach. It is also more likely to pick up Southerly swells wrapping around.
As a rule it is always bigger towards the northern end of the beach (North Rocks), often considerably. At a higher tide this can provide a good shorebreak.
There are usually a couple of good peaks in the middle of the beach off the valley but these are very much dependent on the state of the banks.
At a lower tide there is a fast (closey) right-hander that breaks towards the car park which can get good on the right day.
One of the best waves on the beach is the cafe wave which on a mid tide and big swell can provide some excellent rides.
For the more adventurous (suicidal) there is a reef about 100 metres off shore called the Cowloes. With a decent sized swell, hollow looking lefts can be seen cranking of the rocks here. The Cowloes have been surfed a few times but are still a bit of an unknown and seem to have lots of semi-submurged rocks in the wrong places.
Overall, Sennen is often a safe bet as it's crecsent shape can cope with winds from due east to south westerly. The fact that it's usually twice as big at the northern end means it never really gets maxed out.
These factors make it a wave suitable for all standards - it's also a good learners beach as the waves are often not very steep (plus there is board hire on the beach).
On the negative side it does get pretty busy in the summer.